1. Field of the Invention
This invention relates to an improvement in or relating to an electrophotographic system in which reversal development is performed according to the magnetic brush developing method using a one-component magnetic toner.
2. Description of the Prior Art
The brush-developing method for obtaining a positive photographic image with a one-component magnetic toner ("positive development") is well known. Specifically, it comprises the steps of charging an electrophotosensitive material with electricity, exposing the material to an image to be reproduced to form a corresponding latent image and applying toner particles to the electric charge-remaining portions (unilluminated parts) of the material in the form of a magnetic brush, as is described, for instance, in Japanese Patent Publication No. 56(1981)-2705.
In contrast to this, in a brush-developing method to obtain a negative photographic image ("negative development"), after forming an electrostatic latent image in an electrophotosensitive material, a one-component magnetic toner is applied to the electric charge-free portions (illuminated parts) of the material in the form of a magnetic brush (the same bias voltage being applied both to the electrophotosensitive material and to an associated developing sleeve). This reversal development is described in Japanese Patent Publication No. 56(1981)-2705 and Japanese Unexamined Patent Publication Nos. 52(1977)-146243, 53(1978)-112740. 53(1978)-115299 and 54(1979)-98248. The present inventor used an electrophotosensitive material consisting of pulverized zinc oxide dispersed in an appropriate binder, and carried out the "reversal development". He found that the reproduced images showed much more fog and worse contrast than positive-developed image.
Japanese Unexamined Patent Publication No. 55(1980)-134864 discloses a magnetic brush reversal developing method and apparatus using a one-component magnetic toner to obtain a reproduced image having no fog and good contrast. In carrying out a "reversal development" in the form of a magnetic brush according to the invention under this patent application, an electrophotosensitive material is charged with electricity to a voltage which is relatively low when compared with its saturation voltage (at which the electrophotosensitive material is charged to its full capacity), for instance to a voltage as low as 60 percent of its saturation voltage (non-saturated voltage), the charging voltage being of the same polarity as the polarity of electric charges on the electrophotosensitive material and of equal or somewhat higher value than the potential at the surface of the material. This reversal developing method enables the formation of a photographic image of the same quality (no fog and good contrast) as a photographic image obtained according to the positive developing method. The disclosed method, however, must use a scorotron, which necessitates two separate power supplies allotted for corona wires and for control grids for controlling the amount of electric charge from the corona wires. Accordingly, the apparatus is expensive and large in size. Still disadvantageously, compared with a corotron, it takes a scorotron much time to raise its charging voltage high enough to assure good development and, therefore, development cannot be expedited. In an attempt to expedite development, there has been proposed the use of a scorotron in combination with a corotron, as described in Japanese Unexamined Patent Publication No. 55(1980)-144260. This, however, is not a complete solution to the problem as mentioned above, and what is worse is that it increases the complexity and size of the apparatus as a whole.
In an attempt to improve the quality of a reversal photographic image obtained according to the magnetic brush reversal developing method to the quality of a positive photographic image obtained according to the magnetic brush positive developing method, the present inventor carried out the following experiments:
There were used a magnetic toner used having an electric resistivity of 10.sup.10 .OMEGA..multidot.cm. and a coercivity of 300 oersteds; an electrophotosensitive material consisting a pulverized zinc oxide dispersed in an appropriate binder; and a developing roll comprising 8 magnet pieces each having a strength of 730 gauss and an electric-conductive sleeve enclosure 32 centimeters in diameter. While the sleeve enclosure was kept stationary, the 8-magnet assembly was rotated 1000 times per minute. Two electrophotosensitive sheets were used. One sheet was charged up to saturation voltage with a corotron, whereas the other sheet was charged up to a non-saturation voltage with a scorotron for the purpose of providing a standard of comparison. The charging voltages of these sheets are given in Table 1. After being exposed to an image to be reproduced, the two electrophotosensitive sheets were passed under the developing sleeve at the speed of 7.5 centimeters per second. This experimental revealed that: as regards non-saturation electrostatic charging with the scorotron a fogless photographic image was obtained by applying to the toner on the sheet a bias voltage of substantially the same value as the voltage appearing on the surface of the sheet. As regards saturation electrostatic charging with the corotron a foggy image was obtained when a bias voltage equal to the voltage appearing on the surface of the sheet was applied to the toner attached to the developing sleeve in, for instance, an environment in which the temperature was 17.degree. C. and the humidity was 40%. A fogless photographic image, however, was obtained by applying a much higher bias voltage to the toner on the developing sleeve. The value of bias voltage appropriate for total prevention of fog in the resultant photographic image depends greatly on the temperature, humidity and other environmental factors. In a low-temperature and low-humidity environment, fogless photographic images could not be obtained without applying a voltage to the toner that was very high compared with the surface voltage of the sheet.
In contrast to this, the appropriate bias voltage in case of saturation electrostatic charging with the corotron is substantially independent of the temperature, humidity and other environmental factors.
The results of the experiment are summarized in Table 1.
TABLE 1 __________________________________________________________________________ Non-saturation electrostatic Saturation electrostatic charging with a scorotron charging with a corotron Environment Bias voltage Electrostatic Bias voltage Electrostatic Temperature Humidity appropriate for charging appropriate for charging (.degree.C.) (%) preventing fog (V) voltage (V) preventing fog (V) voltage (V) __________________________________________________________________________ 17 40 -220--260 -250 -700--1300 -450 28 70 -210--250 -250 -400--600 -430 __________________________________________________________________________
In respect of the non-saturation electrostatic charging with a scorotron, Table 1 shows that the bias voltage appropriate for preventing the appearance of fog on a photographic image is independent of the surrounding conditions, and is almost equal to the surface potential (electrostatic charging voltage) on the electrophotosensitive sheet, although showing a slight variation. On the other hand in respect of the saturation electrostatic charging with a corotron, which is employed in the present invention, the appropriate bias voltage greatly varies with the surrounding conditions, although the electrostatic charging voltage remains substantially at the same value. Specifically, the appropriate bias voltage changed from -400--600 volts to -700--1300 volts when the temperature and humidity of the surroundings changed from 28.degree. C. to 17.degree. C. and from 70% to 40% respectively, while the electrostatic charging voltage change only from -430 volts to -450 volts.
The present inventor carried out another experiment as follows:
An electrophotosensitive material of zinc oxide was put first in a relatively high temperature-and-high humidity environment, and then it was shifted to a relatively low temperature-and-low humidity environment (17.degree. C. and 40%), in which the electrophotosensitive material was subjected, in rapid succession, to electrostatic charging, exposing and developing. The lower limit of the range of appropriate bias voltage fell to about 150 volts below the lower limit of the range of appropriate bias voltage in the case of an electrophotosensitive material kept throughout in low temperature-and-low humidity surroundings (17.degree. C. and 40%).
From these experiments the present inventor was convinced that the characteristics of an electrophotosensitive material vary greatly with the temperature and the humidity of the surroundings, and that the bias voltage appropriate for preventing the appearance of fog in a photographic image is relatively large in a relatively low temperature-and-low humidity environment whereas the appropriate bias voltage is small in a relatively high temperature-and-high humidity environment.
The main cause for dependence of the appropriate bias voltage on the temperature, humidity and other surrounding conditions appears to be the dark decay of the electrophotosensitive material and the rise of the electric resistivity of the conductive support layer of the electrophotosensitve material at a relatively low humidity, thereby causing a potential gradient to appear on the even-charged surface of the material. The present inventor carried out various experiments in which: the length of time from electrostatic charging to developing was varied, and use was made of different electrophotosensitive sheets backed with conductive support layers of resistivity little dependent on the varying surrounding conditions, as for instance metal layer or carbon-dispersed layer. From these experiments the present inventor has concluded that there must be causes for variation in the appropriate bias voltage other than those mentioned above.
As seen from Table 1, an electrophotosensitive material electrostatically charged to its saturation is characterized in that its appropriate bias voltage is much different from the charging voltage, and the relatively wide range over which the appropriate bias voltage can vary is characteristic of an electrophotosensitive material electrostatically charged up to saturation.